1. Field of the Invention
This invention relates to an offset type grindstone suitably used to grind hard, fragile materials, such as concrete, stones and the like, and more particularly to a grindstone having a so-called super-grain grindstone portion formed of diamond grains or cBN grains and provided on a grindstone portion mounting section of an offset type steel substrate.
2. Description of the Background
An offset type grindstone formed by fixing so-called super-grains, such as diamond grains or cBN grains and the like on a metal substrate by electrodeposition or a metal bond-sintering method has heretofore been widely used as a tool for subjecting hard, fragile materials, such as stones and concrete to a grinding operation.
A general purpose grindstone of an electrodeposition method which is being manufactured commercially at present is formed by fixing super-grains densely on a surface of an offset type base metal (metal substrate) by electroprecipitation plating of nickel. On the other hand, a grindstone of a metal bond-sintering method is formed by sinter-combining super-grains, such as diamond grains and the like on a surface of an offset type base metal in the same manner by metal bonding.
Since the above-mentioned grindstone of an electrodeposition method has a very large number of grains fixed densely on a grindstone portion, the flat-grinding characteristics thereof with respect to a material to be ground, displayed during a grinding operation are excellent, and a beautiful finished surface is obtained easily. However, this grindstone is cursed by its structure having densely fixed grains, i.e., the height of projecting grains is small, so that actuating depth of each grain with respect to a material to be ground becomes small. Therefore,chips occurring become finer to cause the surface of the grindstone to encounter a phenomenon of being clogged therewith, and this constitutes a primary factor of lowering a grinding speed greatly.
In the case of a grindstone of a metal bond-sintering method, it is possible to reduce an area of a free end which contacts in design a material to be ground of the grindstone, as well as a concentration (compounding ratio) of the grains. Accordingly, it is comparatively easy to obtain an excellent grinding speed but a new problem occurs, i.e., the flatness of a finished surface becomes defective due to a large cutting depth of the grains with respect to a material to be ground. Under the circumstances, attempts are being made to hold down an excessive intrusion of the grains into a material to be ground, by increasing the area of a free end of a grindstone portion or forming the grindstone portion to a spacing material-buried structure for the purpose of improving the flat grindability of the grindstone portion with respect to a material to be ground.
A very high grinding speed and a high flat grindability thus have an antinomic relation, and a worker is pressed under necessity to select a grindstone among grindstones designed in accordance with different purposes, and use selected grindstones for their respective purposes. Therefore, even when such kind of work that is other than work in which a grinding operation is carried exclusively, and in which a surface of a small area is subjected to from a rough grinding operation to a finishing grinding operation in a short period of time, it is necessary to prepare a roughing grindstone and a finishing grindstone in advance, and forward the work by attaching these grindstones in due order to a power tool, such as a disc grinder, etc. in accordance with the progress of the work. Under the circumstances, improving a work efficiency and reducing the cost have been demanded.
In the case of a grindstone of a metal bond-sintering method, a metal-bonded grindstone having a large bulk and heavy weight also has a structure formed by fixing the grindstone to a base metal by welding or brazing. Therefore, the securing the rigidity of the base metal is demanded in view of the construction of this grindstone, and the thickness of the base metal is necessarily set large. Consequently, when a grinding operation, especially, the grinding of a wall surface and a ceiling is carried out by mounting this grindstone on a power tool, such as a disc grinder and the like as a hand tool, a worker""s fatigue increases due to a load ascribed to the weight and a gyroscopic effect of the grinder, and a high degree of skill of a worker is required.
As described above, a related art grindstone has to sacrifice either the very high grinding speed or the high flat grindability. It has strongly been demanded from the view point of the cost and work efficiency that a grindstone capable of attaining the two characteristics be developed. The present invention has been made in view of the above circumstances, and provides a grindstone capable of solving such problems and attaining a very high grinding speed and a high flat grindability at once.
The grindstone according to the present invention made so as to solve the above-mentioned problems is an offset type grindstone having a circular metal substrate provided in a central section thereof with a recess having a hole used to mount the grindstone on a power tool, and a grindstone portion including one layer of either diamond grains or cBN grains and molded in one body so as to extend continuously in the direction of an outer circumference of a front surface of the recess, in which grindstone controlling the concentration (the number of grains per unit area: a compounding ratio) of the grains on the grindstone portion so that the concentration is high in a region of an outer circumferential end section thereof, and lower in the other region thereof than that in the mentioned high-concentration region constitutes the characteristic structural requirements.
To be more exact, the concentration of the grains on the part of the grindstone portion which corresponds to an outer circumferential end section of the circular metal substrate and the concentration thereof in a region thereof extending from the outer circumferential end section toward the center of the substrate by a distance in the vicinity of 0 to 10 mm, preferably 0 to 5 mm is set high so that the concentration is in the range of 50 to 1600 (grains/cm2) while the concentration in the other region of the grindstone portion is set in the range of 5 to 400 (grains/cm2) which is lower than that in the mentioned highly-set regions.
In the offset type grindstone in another example of the present invention, the concentration of the grains on the grindstone portion is set high in a region of an outer circumferential end section as mentioned above, slightly high in the innermost circumferential section of the grindstone portion and in a region extending from the innermost circumferential section toward an outer circumference by within 0 to 10 mm, and lower in the other region than that of any of the mentioned highly-set regions. This also constitutes one of the characteristic structural requirements.
Among the methods of fixing grains to a metal substrate during the formation of a grindstone portion, a furnace brazing method using brazing filler metal is selected as the best method.
In the grindstone according to the present invention, a grindstone portion is provided preferably so as to incline from a substantially radially central part thereof toward an outer circumference thereof. It is also preferable that at least plural stripes of non-grain-fixed regions be provided on the grindstone portion of the grindstone. Providing at least not less than one through hole in a region other than the grindstone portion-fixed part of a circular metal substrate also constitutes one of preferred modes of embodiment.